1. Field of the Invention
The present invention relates to a semiconductor light emitting element such as an LED (light emitting diode).
2. Description of the Related Art
As to LEDs formed of AlGaInP-based material, the band gap of the light emitting layer is larger than the band gap of a GaAs substrate used in crystal-growth. Hence, part of light directed toward the light-extraction-surface side out of the light emitted from the light emitting layer can be extracted, but light directed to the GaAs-substrate side is then absorbed by the GaAs substrate. As an element structure to solve this problem, there is known a so-called stuck-together structure where a light reflecting film is formed on a surface opposite the light extraction surface of the semiconductor film. This light emitting element is produced by, for example, forming the semiconductor film made of AlGaInP-based material on a GaAs substrate and then providing a light reflecting film made of a metal of high reflectance on the surface of the semiconductor film and sticking a support substrate onto the light reflecting film and then removing the GaAs substrate. In an LED of this configuration, light directed toward the opposite side from the light extraction surface is reflected by the light reflecting film and emitted outside, and thus the light extraction efficiency of the LED is improved.
Meanwhile, light incident at an angle greater than or equal to the critical angle relative to the interface between the semiconductor film and the ambient medium such as air or resin is totally reflected and thus cannot be extracted outside. Light which cannot be extracted outside is repeatedly reflected inside the semiconductor film and attenuated. The intensity of light propagating inside the semiconductor film decreases exponentially with the propagation distance (optical path length). For example, if an AlGaInP-based semiconductor film having a refractive index of 3.3 is encapsulated in resin having a refractive index of 1.5, then the critical angle is 27°, and the reflectance at the interface between the semiconductor film and the resin is about 15%. Thus, the proportion of light which can be extracted outside is limited to about 4.5%.
Accordingly, there is known a light extracting structure formed by making the light extraction surface of the semiconductor film rough. With this structure, light directed to the light extraction surface is then scattered and diffracted by protrusions/hollows in the surface of the semiconductor film, thereby reducing the amount of light totally reflected at the interface between the light extraction surface and the ambient medium, and thus the light extraction efficiency can be improved.
Further, among so-called vertical light emitting elements where with an electrode being provided on each of the upper and lower surfaces of the semiconductor film, a current is made to flow in the thickness direction of the semiconductor film, there exists a known light emitting element where the electrode on the upper surface side and the electrode on the lower surface side are placed so as not to overlap each other when they are projected onto a plane parallel to the principal surface of the semiconductor film (that is, the surface perpendicular to the laminating direction of the semiconductor film). This electrode configuration is referred to as counter-electrodes. With this electrode configuration, current spread in directions in the principal surface of the semiconductor film is promoted, so that the uniformity of the current density distribution and the light emission brightness distribution can be improved. Note that the counter-electrodes are usually used in combination with the above stuck-together structure.
Examples of the prior art are as follows:    Japanese Patent Kokai No. 2007-227895 (Patent Literature 1),    Japanese Patent Kokai No. 2008-60331 (Patent Literature 2), and    Japanese Patent Kokai No. 2010-192709 (Patent Literature 3).